1. Field of the Invention
The present invention relates to a solar cell module having excellent weather resistance. More particularly, the present invention concerns an improved solar cell module in which at least the light receiving surface of a photovoltaic element (or solar cell) in the solar cell module is coated with a composition containing vinylidene fluoride copolymer and acrylic resin for achieving a desirable photoelectric conversion efficiency over a long period of time even in a severe environment.
2. Related Art
A number of thin film solar cells have been proposed. An amorphous silicon thin film solar cell (a-Si thin film solar cell) is representative of these thin film solar cells. As the a-Si thin film solar cell, there has been known a solar cell having a construction in which an a-Si semiconducting film as a photoelectric conversion element is provided on a conductive substrate made of a metal layer and a transparent conductive layer is provided on the semiconducting film. In the case where the a-Si thin film solar cell having such a construction is used as a power supplying means, differently from a solar cell of a type in which a glass plate is used as a substrate, a light incident surface of the a-Si thin film solar cell is provided with a means for protecting the surface, that is, a transparent coating material. As the protective means, there is known a construction in which a transparent surface resin layer composed of a fluoride polymer thin film such as a fluororesin film or fluororesin paint is provided on the outermost surface side, and a filler composed of various transparent thermoplastic organic resins is provided below the surface resin layer.
More specifically, as the above surface resin layer, there is used a fluororesin film such as a tetrafluoroethylene-ethylene copolymer film or a polyvinyl fluoride film. The above mentioned filler is interposed between the surface resin layer and the solar cell. As the filler, there is used a resin such as EVA (ethylene-vinyl acetate copolymer) or butyral resin. A back face film is provided on the back face of the above conductive substrate of the solar cell by way of the filler. As the back face film, there is used a nylon film, an aluminum laminated tedlar film, or the like. In addition, the practical solar cell module has a supporting member provided under the back face film by way of the filler.
Accordingly, with respect to the filler interposed between the surface resin layer and the photovoltaic element, the filler interposed between the conductive substrate and the back face film, and the filler interposed between the back face film and the supporting member, each of them requires an adhesive function, and a function of protecting the outside of the photovoltaic element in terms of scratch and impact resistance.
However, when used in the outdoors for a long period of time (for example, 20 years), the prior art solar cell module having the above coating material composed of the surface resin layer and the filler presents the following disadvantage: namely, unsaturated double bonds are present in the main chain of the filler resin and they are conjugated; consequently, the solar cell module exhibits light absorption in the ultraviolet region or in the visible region having a wave length longer than that of the ultraviolet region. In other words, the prior art solar cell module has a disadvantage particularly in terms of weather resistance.
To solve the above-described disadvantage of the prior art solar cell module, Unexamined Japanese Patent No. SHO 58-60579 (hereinafter, referred to as "reference 1") has proposed a technique in which an ethylene series copolymer resin containing a coupling agent and an organic peroxide is used as the resin for the filler of the coating material of the solar cell module. According to the description of the reference 1, as the ethylene series copolymer resin, there may be used ethylene-vinyl acetate copolymer (hereinafter, in some cases, referred to as "EVA") containing vinyl acetate in an amount of about 40 wt % or less, preferably, of from 20 to 40 wt %. Reference 1 further describes that the above EVA is excellent in adhesion to a white white board of polyvinyl fluoride sheet, and in flexibility.
However, to improve the weather resistance of the above EVA, only an ultraviolet ray absorber or hindered amine is added thereto, and accordingly, even the solar cell module using the improved EVA cannot be expected to have good weather resistance over a long period of time (for example, 20 years). The EVA also presents another disadvantage in that it easily yellows, resulting decreased light transmittance, thereby reducing the conversion efficiency of the solar cell module.
This becomes conspicuous when the solar cell module is used at a high temperature, for example, in the case of a module integrated with a roof member. Namely, it is known that the filler composed of EVA is more likely to experience yellowing at a module temperature of 80.degree. C. or more. In general, the EVA used for the solar cell module contains stabilizer additives including an ultraviolet ray absorber, a light stabilizer, and an oxidation inhibitor; however, these additives tend to be volatilized within 10 years or to be bled-out or decomposed.
As a resin for the filler of the coating material of the solar cell module, butyral resin different from the above-described resin has been known. The butyral resin is relatively good in weather resistance; however, it is relatively high in hygroscopicity, which causes a problem that when a solar cell in the module has a damaged portion, moisture is easier to permeate into the module through the damaged portion. Specifically, there often occurs a phenomenon wherein, due to the effect of the moisture thus permeated and the electric field of the solar cell, metal constituting a collecting electrode is repeatedly ionized and precipitated, and it grows in the damaged portion. The progress of this phenomenon (reaction) leads to a problem in producing short-circuits in the solar cell or obstructing the effective collection of separated charges, resulting in reduced conversion efficiency. The butyral resin also exhibits irreversible devitrification which significantly reduces transparency (transparency is not returned even by drying), which is a critical defect for the solar cell module.
Other than the above resins, fluororesin has been proposed for the filler of the coating material of the solar cell module. For example, Examined Japanese Patent No. HEI 4-76229 (hereinafter, referred to as "reference 2") describes a construction in which a CdS/CdTe type solar cell module is provided with a protective film composed of a derivative of a resin containing a perfluoroalkylene group and active hydrogen. In reference 2, as the resin containing a perfluoroalkylene group and active hydrogen, there is proposed a resin product (trade name: Lumiflon) produced by Asahi Glass Co,, Ltd. According to the description of reference 2, the resin product is a fluorine-containing polymer which has a perfluoroalkylene group and pendant active hydrogen, more specifically, an 0H group, and which generally has a number-average molecular weight in the range of 20,000 to 80,000. This reacts with a compound having a melamine group or isocyanate group to produce the derivative (cross-linked polymer) of the above resin. Reference 2 further describes that the above LUMIFRON is cross-linked by isocyanate or resol type phenol resin, to obtain a protective film having excellent moisture resistance.
Incidentally, in the coating technique using the above cross-linked resin, described in reference 2, the protective film is required to be positioned on the outermost surface of the solar cell module. Specifically, the pot life of the resin after the above cross-linking agent is added is generally short, and actually, the pot life is prolonged by protecting the isocyanate by adding a blocking agent.
However, as described above, in the case of adopting a coating material structure in which the surface resin layer is laminated on a specified resin, dissociation and volatilization of the blocking agent upon cross-linking of the resin is suppressed by the presence of the surface resin layer, which obstructs the progress of the cross-linking reaction. Even when the above resin is cross-linked before the surface resin layer is laminated thereon for accelerating the dissociation and volatilization of the blocking agent, the cross-linked resin is poor in stickiness and adhesiveness, and thereby the lamination is difficult to be performed. In the case of using melamine as the cross-linking agent, no effective blocking agent is known; consequently, such a cross-linked resin is required to be used on the outermost surface of the solar cell module. Incidentally, the surface hardness of the above cross-linked resin is generally as low as about B to H in pencil hardness. The cross-linked resin having such a surface hardness is easily damaged by contact with sand, dust, and the like present outdoors, and collects contamination and refuse in the damaged portions, thereby being shielded from the sunshine. As for the lamination of the cross-linked resin, while being affected by the laminating manner, only the coating of a paint presents a problem of generating pin holes and the entrapment of refuse thereby causing moisture or oxygen to easily permeate the photovoltaic element (solar cell). In view of the foregoing, there has been not found a filler made of organic material used for a coating material capable of maintaining the solar cell module at a high level of weather resistance and moisture resistance.
Incidentally, coating material constituted of glass has been considered to be most desirable for solving the problem in terms of yellowing and hygroscopicity of the material, and a technique has been proposed in which the solar cell is encapsulated with glass. This technique, however, has a problem that the coating using glass is poor in flexibility, impact resistance, high in weight, and cost reduction; accordingly, it is undesirable for a solar cell having a large area which is used outdoors.